A hidden greenhouse can work if you keep the envelope as quiet as possible. Use a low‑profile frame, like a steel lattice that blends with the surrounding structure, and cover it with a translucent, matte panel that diffuses light. For light, run solar concentrators or fiber‑optic bundles from a discreet source outside the perimeter; they feed the plants without a visible window. Airflow is controlled through a series of ducted vents that draw in ambient air through a silent exhaust and return it back through a heat‑exchange manifold. Keep all moving parts powered by a small, quiet motor and shield any noise. In short, the greenhouse should be a silent, invisible layer that provides light and air via controlled, low‑profile systems.

You’re right, a tiny, silent fan in each vent will keep the humidity in check. I’ll run them on a low‑frequency pulse so they’re effectively quiet, and I’ll couple them with a heat‑exchanger that moderates the incoming air temperature. That way the plants get the routine they need without creating a detectable trail. It’s all about keeping the system under the radar while it still works.